The present invention relates to golf balls and, more particularly, the invention concerns novel golf ball covers made from ionomer resin formulations containing high levels of metal stearates. The new low-cost formulations produce, when utilized for the production of golf ball covers, golf balls exhibiting similar or enhanced travel distance and/or playability properties while maintaining the durability characteristics necessary for repetitive play.
Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability, and flight characteristics, various ionomeric resins sold by E.I. DuPont de Nemours & Company under the trademark "Surlyn.RTM." and more recently, by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademarks "Escor.RTM." and the tradename "Iotek", have become the materials of choice for the construction of golf ball covers over the traditional "balata" (trans polyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability necessary for repetitive and/or continuous play.
Ionomeric resins are ionic copolymers of an olefin, such as ethylene, and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. In some instances, an additional softening comonomer such as an acrylic ester can also be included to form a terpolymer. The pendent ionic groups in the ionomeric resins interact to form ion-rich aggregates contained in a non-polar matrix.
Broadly, the ionic copolymers generally comprise one or more .alpha.-olefins and from about 9 to about 20 weight percent of .alpha., .beta.-ethylenically unsaturated mono- or dicarboxylic acid, the basic copolymer neutralized with metal ions to the extent desired. Suitable olefins for use in preparing the ionomeric resins include ethylene, propylene, butene-1, hexane-1, and the like. Unsaturated carboxylic acids include acrylic, methacrylic, ethacrylic, .alpha.-chloroacrylic, crotonic, maleic, fumaric, itaconic acids, and the like.
The ionomeric resins utilized in the golf ball industry are generally copolymers of ethylene with acrylic (i. e. Escor.RTM. or Iotek) and/or methacrylic (i.e. Surlyn.RTM.) acid. In addition, two or more types of ionomeric resins may be blended into the cover compositions in order to produce the desired properties of the resulting golf balls.
In the ionomeric resins, metal ions, such as sodium, zinc, magnesium, lithium, potassium, calcium, etc., are used to neutralize some portion of the acid groups in the ionic copolymers resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e. improved durability, etc., for golf ball cover construction over balata. However, the advantages gained in increased durability have been offset to some degree by the decreases produced in playability. This is because although the ionomeric resins are very durable, they tend to be very hard when utilized for golf ball cover construction, and thus lack the degree of softness required to impart the spin necessary to control the ball in flight.
As a result, while there are currently more than fifty commercial grades of ionomers available from DuPont and Exxon with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e. relative content of ethylene and methacrylic and/or acrylic acid groups), the degree of neutralization, etc., and wide number of additive ingredients which may be added to improve various characteristics of the cover stock compositions, including reinforcing materials such as glass fibers and inorganic fillers, softening agents such as plasticizers, and other compatible ingredients such as antistatic agents, antioxidants, stabilizers, processing acids, etc., a great deal of research continues in order to develop golf ball cover compositions exhibiting not only the improved impact resistance and carrying distance properties produced by the "hard" ionomeric resins, but also the playability (i.e. "spin") characteristics previously associated with the "soft" balata covers, properties which are still desired by the more skilled golfer.
In addition, golf ball manufacturers are continuously reviewing on a manufacturing cost basis, different compositions and processes for producing golf balls exhibiting similar or improved characteristics at substantially lower costs. This is particularly true with respect to golf ball cover compositions wherein the ionomer resins are relatively expensive (i.e. about $1.75/lb.). Consequently, various additives, such as fillers, have been added to the cover compositions. However, drastic reductions in a number of desired characteristics such as resilience (i.e. C.O.R.) are normally exhibited due to the incorporation of high levels of fillers in the ionomer formulations.
In this regard, two of the principal properties involved in the performance of golf balls are resilience and hardness. Resilience is determined by the coefficient of restitution (C.O.R.) , the constant "e", which is the ratio of the relative velocity of an elastic sphere after direct impact to that before impact. As a result, the coefficient of restitution (i.e. "e") can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision.
Resilience (C.O.R.), along with additional factors such as ball size and weight, clubhead speed, angle of trajectory, and ball aerodynamics (i.e. dimple pattern), generally determine the distance a ball will travel when hit. Since clubhead speed and the angle of trajectory are not factors easily controllable, particularly by golf ball manufacturers, and the ball size and weight parameters are set by the United States Golf Association, the factors of concern among manufacturers are the coefficient of restitution (C.O.R.) and the surface configuration of the ball.
The golf ball's coefficient of restitution (C.O.R.) is a function of the composition of the molded core and of the cover. In balls containing a wound core (i.e. balls comprising a liquid or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings. Although both the core and the cover contribute to the coefficient of restitution, the present invention is directed solely to the coefficient of restitution which is affected by the cover composition.
The coefficient of restitution of a golf ball is generally measured by propelling a ball at a given speed against a hard surface and measuring the ball's incoming and outgoing velocity electronically. As mentioned above, the coefficient of restitution is the ratio of the outgoing velocity to incoming velocity. The coefficient of restitution must be carefully controlled in all commercial golf balls in order for the ball to be within the specifications regulated by the United States Golf Association (U.S.G.A.). Along this line, the U.S.G.A. standards indicate that a "regulation" ball cannot have an initial velocity (i.e. the speed off the club) exceeding 255 feet per second. Since the coefficient of restitution of a ball is related to the ball's initial velocity, it is highly desirable to produce a ball having a sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of hardness (i.e. softness) to produce enhanced playability (i.e. spin, etc.).
The hardness of the ball is the second principal property involved in the performance of a golf ball. The hardness of the ball can affect the playability of the ball on striking and the sound or "click" produced. Hardness is determined as the deformation (i. e. compression) of the ball under various load conditions applied across the ball's diameter (i.e. the higher the compression value, the softer the material) and/or by Shore hardness characteristics (i.e. the lower the Shore hardness value, the softer the material). As indicated in U.S. Pat. No. 4,674,751, "softer" covers permit the accomplished golfer to impart proper spin. This is because the softer covers deform on impact significantly more than balls having "harder" ionomeric resin covers. As a result, this allows the better player to impart fade, draw, or backspin to the ball thereby enhancing playability.
Accordingly, the present invention is directed to new ionomer resin formulations containing high levels of metal stearates. It has been found that ionomer resin formulations containing high levels of metal stearates (i.e. about 10-100 parts by weight per 100 parts by weight of the ionomer (pph)) produce, upon molding around solid or wound cores to formulate a cover composition, golf balls exhibiting similar or enhanced distance (i.e. resilience) and playability (i.e. hardness/softness) properties without affecting the ball's durability (i.e. impact resistance, etc.) characteristics.
Along this line, metal stearates are generally useful in a wide variety of industries. They are useful wherever water repellency, internal or external lubrication, anti-agglomeration of particulate solids or viscosity modification (including gelation) is desired. Other inherent performance qualities that make stearates important to industry are their ability to impart flattening to varnishes, lacquers and solvent-based paints; improved suspension of solids in such paints; adhesion and slip to cosmetic powders; and dry lubrication to surfaces and particles. As a result, applications of metal stearates are found in toiletries and pharmaceuticals, cement and concrete, food, greases, inks, metal working and powder metallurgy, paper coating, plastics and rubber, petroleum production and wax products.
In the polymer industry, metal stearates are widely used as a lubricant. The most important of these is calcium stearate, used predominantly in PVC but also in polyolefins, ABS, polyesters, and phenolics. Calcium stearate acts as an internal lubricant in PVC, to promote fusion and modify melt viscosity during extrusion processing. It also acts as a co-stabilizer in many PVC applications when combined with a number of primary heat stabilizers. Other metal stearates, e.g., lead, barium, cadmium, zinc, and magnesium stearate are multifunctional in that they provide both heat stability and balanced lubricity to many PVC formulations when used in combination with other lubricant types. Small amounts of these metal stearates are also used to aid in the processing of polyolefins, ABS, nylon, polyester, and polystyrene. Their use here is generally to aid in metal release, rather than to act as internal lubricants.
While metal stearates are typically used as inert processing additives in rubber and plastic formulations, they are generally used in relatively low amounts (i.e. 0.01 to about 5 pph) as lubricants and/or dispersants to facilitate flow and/or to act as metal and/or mold releasing agents and as inorganic fillers. This is particularly true in the industry of golf ball production, wherein low amounts of metal stearates, such as about 0.01 to about 1.0 pph zinc stearate, have been utilized since at least the middle 1960's to facilitate the flow of ionomer resins in the molding process.
However, the present invention is directed to the discovery that the metal stearates, when added at relatively high levels to ionomer resin formulations, act as an ionic plasticizer". This occurs only when metal stearates in amounts greater than about 10 pph resin, generally from about 10 to about 100 pph resin, more preferably about 25 to about 75 pph resin, and most preferably, about 50 pph resin, are utilized in combination with the known ionomer resin formulations.
Since nearly all fillers produce drastic reductions in C.O.R. values when utilized in relatively high amounts in ionomer resin compositions, it was very surprising that the ionomer formulations of the present invention having high levels of metal stearates did not demonstrate these same negative results.
In addition, the low specific gravity of the various metal stearates such as zinc stearate, also allows for high loadings of filler without producing an illegally heavy golf ball as in the case of the HiSil.RTM. fumed silica produced by PPG, Pittsburgh, Pa., which is generally regarded as the best white reinforcing filler in many rubber and plastics applications including the field of golf ball construction.
Consequently, the present invention is directed to the use of relatively high levels (greater than 10 pph resin) of metal stearates in ionomer resin compositions for the purpose of reducing cost and, in some circumstances, enhancing the C.O.R. and/or hardness (i. e. softness) values of the resulting cover compositions without affecting the cover's desired durability (impact resistance, etc.) properties. As a result of the improved C.O.R. values produced by the new ionomer resin formulations of the invention the balls exhibit enhanced travel distance. In addition, the new cover compositions produce, upon molding around solid or wound cores, golf balls exhibiting a sufficient amount of hardness (i.e. softness) to improve the playability characteristics of the balls.
These and other objects and features of the invention will be apparent from the following description and from the claims.